1. Field of the Invention
The present invention relates to an image forming apparatus such as an ink jet printer for effecting printing by discharging liquid ink onto a printing medium, and a discharged sheet stacking apparatus associated with such an image forming apparatus and having a function of preventing a print surface of a printed sheet (before ink fixing) from abrading, and a control method therefor.
2. Related Background Art
Recently, ink jet printers have widely been applied to personal computers, word processors and facsimiles because of their high speed printing abilities and high print quality.
A typical construction of an ink jet printer is shown in FIG. 10.
In the typical ink jet printer, a printing operation is effected in such a manner that a carriage 1 on which a head for discharging ink in a z-direction is mounted scans a sheet 2 (being conveyed) along a y-direction perpendicular to a sheet conveying direction x.
The sheet 2 is intermittently conveyed by a conveying roller 3 by an amount corresponding to a length printed by one scanning of the carriage 1, and the printing operation is continued by repeating the scanning of the carriage 1 and the sheet conveyance of the conveying roller 3.
The entire printing time is mainly determined by a sheet conveying speed and a scanning speed of the carriage 1.
Further, the sheet conveying speed is determined by the ability of a sheet conveying mechanism and the ability of a drive source therefor.
Although depending upon the abilities of a scanning mechanism and a drive source therefor, the scanning speed of the carriage 1 is mainly determined by the property of ink regarding the head, and ink discharging ability determined by heaters (discharging means) mounted on the head and other elements, i.e., ink discharging amount per unit time.
Further, in the ink jet printer, an operator can select a print mode.
Print modes which are normally provided in the apparatus and which can be selected by the operator include a high speed print mode for improving the printing speed, a high image quality print mode which attaches importance to the printing speed, and a high image quality print mode which attaches importance to image quality. And, in some printers, these modes are divided into several stages.
In the high speed print mode, it is required that the scanning speed of the carriage 1 be increased while utilizing the ink discharging ability of the head to the maximum.
To achieve this, for example, ink discharging numbers per one scanning of the carriage 1 are usually reduced to 1/2 to cope with the high scanning speed of the carriage 1.
As a result, in the high speed print mode, although the resolving power of image and print density are decreased in comparison with other modes, the important high printing speed feature can be obtained.
On the other hand, in the high image quality print mode, since the resolving power of image can be improved and overlapping print is effected to reduce unevenness of sheet conveyance, the scanning speed of the carriage 1 is decreased and the scanning numbers are increased, with the result that the printing speed tends to be decreased.
Further, in the ink jet printer, since the printing is effected by discharging the liquid ink onto the sheet, if a succeeding printed sheet is stacked to contact a print surface of the preceding printed sheet before the ink on the surface of the preceding printed sheet is dried and fixed, the print surface of the preceding sheet will be contaminated by ink.
To avoid this, in the conventional ink jet printer, a heater is incorporated in the apparatus to promote the drying of the print surface of the sheet or a discharged sheet stacking device is provided as a means for preventing the succeeding sheet from overlapping or abrading before the ink printed on the surface of the preceding sheet is dried and fixed in the continuous printing.
FIG. 11 is a perspective view of a conventional ink jet printer having a discharged sheet stacking device.
As shown in FIG. 11, in a printer 4, a sheet 2 is conveyed by an internal sheet conveying mechanism (not shown) as the printing operation progresses and is discharged from a sheet discharging port 5.
The sheet 2 discharged from the sheet discharging port 5 temporarily rests on two movable support members 7 arranged in a discharged sheet stacking portion (containing portion) 6 which is disposed at a downstream side of the sheet discharge port 5 in a sheet discharging direction and on which the printed sheets are ultimately stacked. The support members 7 can be moved between a first position where the sheets are held by the support members 7 and a second position where the sheets are not held by the support members 7.
FIG. 12 is a front view of the discharged sheet stacking device.
Since the sheet 2 is held by the support members 7 in a spaced relationship to a previously printed sheet 8 stacked on the discharged sheet stacking portion 6 until a time period required for drying the ink on the previous sheet 8 elapses, the sheet 2 is prevented from overlapping or abrading with respect to the previous sheet 8.
After the time period for drying the previous sheet 8 has elapsed, two support members 7 are spaced apart from each other by a distance greater than a width of the sheet by a cam or other means (not shown) for driving the support members 7, with the result that the held sheet 2 is dropped and is stacked on the previous sheet 8 resting on the discharged sheet stacking portion 6.
Since the sheet is stacked onto the discharged sheet stacking portion 6 after the time period required for drying the ink (time period for printing one sheet or more) has elapsed, the contamination of the print surface of the sheet due to overlapping or abrading between the sheets can be prevented. If a further drying time period is required, the printing of the next sheet can be stopped while holding the sheet 2.
The time period required for drying the ink is closely associated with print density on the sheet. Namely, the higher the print density (i.e., the greater the ink amount) the longer the fixing time (i.e., longer than the drying time).
On the other hand, if the density is reduced by decreasing the resolving power, the amount of ink used also decreases to shorten the fixing time.
However, in the above-mentioned conventional technique, the following problems arose.
In case of the conventional ink jet printer and the discharged sheet stacking device having the above-mentioned constructions, an operation of the discharged sheet stacking device is incorporated in the series of printing operations so that the discharged sheet stacking device performs holding, releasing and stacking of the printed sheet in any mode.
Accordingly, in comparison with the printer having no discharged sheet stacking device, the printing speed tends to decrease due to the presence of the operation of the discharged sheet stacking device, and, particularly in the high speed print mode, the requirement of the operator cannot be satisfied.